1. Field of the Invention
This invention relates to a thread forming fastener. It relates especially to a self-tapping screw that is particularly adapted for use with a nut anchor of a pliable material. By the term “nut anchor” is meant any separate nut or any anchor or support to which the fastener may be secured.
2. Background Information
It is generally recognized that self-tapping fasteners that screw into pliable materials such as plastics, soft metals, alloys, etc. need to address several basic requirements, namely:                during assembly, they should develop a low thread tapping torque when forming the mating thread in the associated nut or support structure,        upon tightening, the fasteners should be capable of maintaining a relatively high resistance to thread stripping torque,        the assembly of the fasteners and nut anchors should be capable of maintaining joint integrity under externally applied forces, and        the fasteners should be able to maintain a reasonable resistance to joint loosening that can be caused by plastic flow or creep of the pliable material comprising the nut anchors into which the fasteners are screwed.        
Many such fasteners used today have thread designs of a triangular cross-sectional geometry that are either symmetrical about a line that is perpendicular to the screw axis, are constructed to be asymmetrical to this same perpendicular or are constructed as stacked trapezoidal shapes. There also exist asymmetrical thread profiles that depart from the aforesaid triangular or trapezoidal forms, and develop forces within a nut anchor material that create and encourage anchor material flow in a manner that increases thread flank material contact along the mating trailing faces of the screw thread profile; see my U.S. Pat. No. 5,061,135.
As shown in FIG. 1, the aforesaid patented fastener has a shank shown generally at 2 comprised of a cylindrical core 3 and a thread 4 generated on a helical form around core 3, the thread having pitch 4′. The trailing face 5 of the screw 4 profile lies at an angle α, preferably 10°-15°, to the thread diameter which diameter is perpendicular to the longitudinal axis of core 3. The thread profile also has a leading face 6 with a curved portion 6a extending from core 3 to a point 7 on the leading face where it transitions to a linear portion 6b with that portion lying at an angle β relative to said diameter of 17° to 25° and extending to a tip 8. The vertical distance from point 7 to tip 8 is in the range of 23-27% of the thread height 9. Also, the magnitude of the radius 10 of the curved portion 6a is 0.22 to 0.30 times the nominal screw diameter or size.
A fastener having the aforesaid screw thread geometry creates improved performance conditions by generating material flow of the nut anchor material. However, that material flow is not as smooth as might be desired and that prior screw still has some potential for causing stress cracking of the nut anchor material during insertion of the fastener.